custom-reporter.js

const fs = require('fs');

// const report = {
//     'foo.tsx': {
//         12: ['foo.spec.tsx', 'bar.spec.tsx'],
//         389: ['bat.spec.tsx']
//     }
// };

class reporter {
    constructor() {
        this.coverage = {};
    }

    onTestResult(config, result) {
        Object.entries(result.coverage).forEach(([fileName, stats]) => {
            Object.values(stats.fnMap).forEach((v) => {
                if (!this.coverage[fileName]) {
                    this.coverage[fileName] = {
                        [v.line]: [result.testFilePath]
                    };
                    return;
                }

                if (!this.coverage[fileName][v.line]) {
                    this.coverage[fileName][v.line] = [result.testFilePath];
                    return;
                }

                this.coverage[fileName][v.line].push(result.testFilePath);
            });
        });
    }

    onRunComplete() {
        const lines = [];

        Object.entries(this.coverage).forEach(([fileName, cov]) => {
            Object.entries(cov).forEach(([ln, tests]) => {
                lines.push({
                    fileName,
                    ln,
                    tests
                });
            });
        });

        lines.sort((a, b) => {
            if (a.tests.length > b.tests.length) {
                return -1;
            }

            if (a.tests.length < b.tests.length) {
                return 1;
            }

            return 0;
        });

        const strs = lines.map(
            ({ fileName, ln, tests }) =>
                `**${fileName} (ln${ln}) - ${tests.length}**\n\n\n${tests.join('\n')}`
        );

        fs.writeFile('report.txt', strs.join('\n\n'), 'utf8', () =>
            console.log('done')
        );
    }
}

module.exports = reporter;